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    SubjectRe: [PATCH 2/2] mm,migration: Avoid race between shift_arg_pages() and rmap_walk() during migration by not migrating temporary stacks
    On Mon, May 10, 2010 at 12:41:07PM -0500, Christoph Lameter wrote:
    > On Tue, 4 May 2010, Mel Gorman wrote:
    >
    > > On Sat, May 01, 2010 at 08:56:18PM -0500, Christoph Lameter wrote:
    > > > On Thu, 29 Apr 2010, Mel Gorman wrote:
    > > >
    > > > > There is a race between shift_arg_pages and migration that triggers this bug.
    > > > > A temporary stack is setup during exec and later moved. If migration moves
    > > > > a page in the temporary stack and the VMA is then removed before migration
    > > > > completes, the migration PTE may not be found leading to a BUG when the
    > > > > stack is faulted.
    > > >
    > > > A simpler solution would be to not allow migration of the temporary stack?
    > > >
    > >
    > > The patch's intention is to not migrate pages within the temporary
    > > stack. What are you suggesting that is different?
    >
    > A simple way to disallow migration of pages is to increment the refcount
    > of a page.
    >

    I guess it could be done by walking the page-tables in advance of the move
    and elevating the page count of any pages faulted and then finding those
    pages afterwards. The fail path would be a bit of a pain though if the page
    tables are partially moved though. It's unnecessarily complicated when the
    temporary stack can be easily avoided.

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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